Transistors are an essential component in modern mobile communications devices. Specifically, transistors play a vital role in the transmission and reception of radio frequency (RF) signals in the front end of a mobile communications device. Due to the decreasing form factor of mobile communications devices, the desire for a longer battery life, and support for an increasing number of stringent wireless communications standards, there is an ongoing need for smaller, more efficient transistor devices with improved performance characteristics.
As will be appreciated by those of ordinary skill in the art, one way to improve the performance of a transistor device operating at high frequencies (e.g., radio frequencies) is by using a heterojunction bipolar transistor. At high frequencies, heterojunction bipolar transistors offer many performance advantages over homojunction bipolar transistors. The performance advantages offered by conventional heterojunction bipolar transistors primarily arise due to a wider energy bandgap in the material of the emitter of the device as compared to the energy bandgap in the material of the base of the device. The wider energy bandgap of the emitter material allows for many parameters dictating the performance of the device to be optimized for high frequencies without degrading the current gain of the device.
While heterojunction bipolar transistors generally offer performance advantages at high frequencies, multiple types of heterojunction bipolar transistors may each provide particular benefits. For example, a single heterojunction bipolar transistor, wherein one of either a base-emitter junction or a base-collector junction is a heterojunction, typically achieves desirable direct current (DC) gain at higher supply voltages. However, at lower supply voltages, conventional single heterojunction bipolar transistors generally do not provide high DC gain. On the other hand, a double heterojunction bipolar transistor, wherein both the base-emitter and the base-collector junctions are heterojunctions, typically provide high DC gain at both higher and lower supply voltages. Despite such desirable DC performance, conventional double heterojunction bipolar transistors typically do not offer high RF gain at lower supply voltages.
Therefore, it would be advantageous to employ a heterojunction bipolar transistor capable of providing high DC gain across a wide range of supply voltages while also achieving high RF gain at lower supply voltages.